# std::match_results<BidirIt,Alloc>::operator[]

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< cpp‎ | regex‎ | match results

const_reference operator[]( size_type n ) const; |  | (since C++11)

If n > 0 and n < size(), returns a reference to the std::sub_match representing the part of the target sequence that was matched by the nth captured marked subexpression).

If n == 0, returns a reference to the std::sub_match representing the part of the target sequence matched by the entire matched regular expression.

if n >= size(), returns a reference to a std::sub_match representing an unmatched sub-expression (an empty subrange of the target sequence).

`ready()` must be true. Otherwise, the behavior is undefined.

### Parameters

n | - | integral number specifying which match to return

### Return value

Reference to the std::sub_match representing the specified matched subrange within the target sequence.

### Example

Run this code

    #include <iostream>
    #include <regex>
    #include <string>

    int main()
    {
        std::string target("baaaby");
        std::smatch sm;

        std::regex re1("a(a)*b");
        std::regex_search(target, sm, re1);
        std::cout << "entire match: " << sm[0] << '\n'
                  << "submatch #1: " << sm[1] << '\n';

        std::regex re2("a(a*)b");
        std::regex_search(target, sm, re2);
        std::cout << "entire match: " << sm[0] << '\n'
                  << "submatch #1: " << sm[1] << '\n';
    }

Output:

    entire match: aaab
    submatch #1: a
    entire match: aaab
    submatch #1: aa

### See also

str | returns the sequence of characters for the particular sub-match (public member function)

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